The present invention relates generally to machine control and, more particularly, concerns a method and a system for precision machining or finishing of article surfaces. It finds application, among other uses, in polishing of the semiconductor layer of semiconductor-on-insulator structures.
The present invention will be disclosed in terms of a particular application. However, other applications are disclosed and further applications will be apparent to those skilled in the art. Particular applications were disclosed for convenience of description and without the intention of limiting the invention to any of them.
To date, the semiconductor material most commonly used in semiconductor-on-insulator structures has been silicon, and glass is a common insulator. Silicon-on-insulator technology is becoming increasingly important for high performance thin film transistors, solar cells, and displays, such as active matrix displays. Silicon-on-insulator wafers consist of a thin layer of substantially single crystal silicon (generally 0.1-0.3 microns in thickness but, in some cases, as thick as 5 microns) on an insulating material.
Once the semiconductor-on-insulator structure has been bonded to a thin film of silicon, it is typically necessary to polish the surface of the silicon layer to produce a layer having a substantially uniform thickness, in order to facilitate the formation of thin film transistor (TFT) circuitry on the silicon.
As a specific example, silicon-on-glass (SiOG) substrates are subjected to a machining process that thins the surface film. This is commonly performed by “deterministic polishing,” an abrading process performed by a tool that has a substantially smaller polishing contact zone than the component being machined. This type of process is typically performed today by the use of ultra-precise optical lens polishing machines, a well-known source of which is Zeeko Limited of Coalville, Leicestershire, UK. A machine of this type is disclosed in U.S. Pat. No. 6,796,877, entitled ABRADING MACHINE and issued to Bingham et al. On Sep. 28, 2004. As is typical, precision movement between a machine tool and work piece is provided in three Cartesian coordinates, in order to achieve machining of the entire surface.
The machining tool of the type disclosed in U.S. Pat. No. 6,796,877 may be referred to herein as a bonnet/pad machine, and is illustrated schematically in FIG. 1. The tool 10 has a generally cylindrical body 12 and a working head or bonnet 14 which is internally pressurized to a predetermined pressure. For example, the bonnet may be a partially spherical or bulbous, fiber reinforced rubber diaphragm. A polishing pad 16 is bonded onto the surface of bonnet 14. In operation, the pad 16 is applied to a surface of the component being machined and is rotated about an axis of rotation A, in order to abrade the surface.
Prior to use, the tool must be calibrated to the work piece surface to be machined. In order to do this, the pad 16 is touched to the surface at a number of points in a predetermined pattern. Tool 10 is provided with a positioning mechanism 19 providing precision movement along three axes and the axial movement corresponds to the Z-axis control. In performing the calibration, when the pad 16 is touched to one of the calibration points on the surface, bonnet 14 is moved axially until a predetermined force is sensed by a sensor 18 provided in tool 10. This assures consistency of contact. After a set of calibration points has been taken, tool movement can be controlled to assure that the bonnet will remain in a plane or other appropriate contour corresponding to the intended finished shape of the surface to be machined. In addition, an appropriate axial spacing of bonnet 14 relative to the surface to be polished will be maintained. This is normally an interference spacing that would place the front of the bonnet past the surface of the work piece, causing compression of the bonnet against the surface. The actual machining process is then performed by rotating bonnet 14 and simultaneously moving it in a predetermined scanning pattern along a contour (e.g., a plane) relative to the work piece surface to be machined. Although different scanning patterns are available, the most common pattern is a series of closely spaced parallel lines or a “raster”, similar to the line pattern scanned on a cathode ray tube of a traditional television set.
The requirements for SiOG film thinning are quite stringent. It would be desirable for the final film thickness to be controlled with an accuracy of about ±8 nm. It is known that material removal is approximately linearly proportional to the scan rate of the bonnet and the bonnet rotational speed. However, it is proportional to the square of the polishing spot size, or the area of the pad which actually performs the abrasion. Polishing spot size is controlled by the amount of force between the bonnet and the surface being machined, which results from its interference contact with the surface to be polished. All of these parameters are well understood, and current polishing practice closely controls them.
It has been found that deviations in the rotation of bonnet 14 have a profound effect on material removal. Such deviations could be measured by rotating bonnet 14 and measuring the amount of radial (eccentric) movement, which will be referred to herein as “radial error motion.” It will be appreciated that any eccentricity in pad rotation will make the spot size effectively larger, resulting more material removal than expected, at high rotational speeds and time variable material removal at low rotational speeds. It has been found that a radial error motion of approximately 50 microns may result in a film thickness variability of approximately 15 nm, larger than the total film thickness tolerance. Every effort is made to minimize the combined radial error motion of the bonnet and pad (e.g., by diamond turning and/or cup grinding in situ). However, this radial error motion can rarely be reduced below 30 microns.
It is therefore clear that, in order to achieve the required film thickness control when performing the deterministic polishing with a bonnet/pad type machine, the bonnet spot size must be controlled to tighter tolerances than can be achieved by bonnet truing.